In many telemetry system applications, and in particular in the field of medical devices, the system must ensure the ability to detect data signals in the presence of significant noise. Often the noise may have components within the frequency band of the telemetry signal, making the detection process difficult. It is known in telemetry systems to use window tracking to detect pulses. In such systems, a detection window is created centered around the next expected pulse, to time discriminate against noise and thereby enable examination of the incoming signal. However, generally in such systems the time of the detected pulse is not sharply defined, and the window needs to be long enough to both "see" the pulse and allow for drift in the pulse position. Consequently, it is very difficult to separate out noise from signal in such time-based systems. Other systems have been employed with varying success, but it remains difficult to accurately and reliably receive pulsatile data in a noisy environment. An acceptable receiver, e.g., for frame-based uplink telemetry, using DSP or any other embodiment, must provide a simple yet very reliable method of discriminating the noise likely in the environment in which the system operates.
A problem which comes into play in telemetry systems involving implanted devices is that the carrier is frequently of an unstable and inaccurate nature. In many such systems the carrier is a continuous wave, i.e., a sinusoidal carrier, such that the phase information of the carrier can be retrieved by multiplying it with sine waves and cosine waves (complex demodulation). However, if the type of carrier is a complex multi-frequency wave form, e.g., monopolar chirps, etc., the necessary phase information is not easily retrieved, and an improved form of phase detection is required. Generally, where the telemetry system uses pulsatiles that can be regarded as short spread spectrum RF bursts with wide band signal properties, the receiver must also obtain information about the characteristics of the signal in order to effectively detect it in the presence of noise.
In view of the above, it is seen that what is needed in the art is an improved telemetry system, and in particular, a telemetry system with an improved noise-suppressing telemetry receiver. In particular, the need is to provide demodulation of pulsatile high frequency signals of various forms, e.g., multi-frequency wave forms such as BPSK signals, exponentially decaying sinusoidal signals, etc. In such telemetry systems, pulsatile RF signals are modulated in the transmitter by a data-carrying symbol signal with an accurate symbol rate. This invention uses the inherently accurate symbol rate as a basis for deriving the phase and other characteristics of the transmitted signal, for use in demodulating the RF signals and obtaining the transmitted data.
Further, telemetry receivers for uplinking data for implanted devices such as cardiac pacemakers, can utilize the efficiency and reliability inherently provided by DSP implementation. Examples of such inherent power are seen in cross correlation detection implemented by a finite impulse response (FIR) digital filtering structure, and quadrature demodulation. The potential of DSP based processing in fields such as cardiac pacing systems has been demonstrated. See U.S. Pat. Nos. 5,448,997 and 5,446,246. This invention may utilize the processing power of DSP to enable an improved time discrete system design for suppressing noise and reliably detecting data uplinked from, e.g., an implanted medical device, but also embraces other state-of-the-art embodiments.